There are many industrial applications which would benefit from quick, non-contact measurement of materials inside containers or flowing through pipes. Examples include emulsion preparation, separation processes, agglomeration reactors, milling operations and polymerization. For each of these applications there is a need for measurement of the constituent component ratios and contaminates, or characterization of physical properties such as particle sizes in slurries and emulsions. Similarly, for some applications, only the identity of the enclosed material (e.g., wax vs. oil) needs to be determined.
Various apparatus and methods for composition and particle size measurement utilizing ultrasound are known in the prior art. Several particle measurement systems described in the prior art make use of both attenuation and sound velocity measurements at a few selected ultrasonic frequencies. Sowerby, U.S. Pat. No. 5,569,844, describes a combination gamma ray and ultrasonic system that operates at two discrete frequencies to measure solids loading, particle size and solute concentration in a suspension. Both gamma ray measurements and the two-frequency velocity readings are used in a linear equation to calculate solute concentration in a stirred tank. Similarly, the particle sizes in preset ranges are found by using the two attenuation measurements, the velocity measurements and gamma ray density measurements in another linear equation. The two frequencies are selected to optimize measurement sensitivity for the expected particle distribution. Related International Application Number PCT/AU96/00362, Publication Number WO 97/00436, describes the combined use of gamma ray measurement and sound velocity at several discrete frequencies to measure particle size alone. Operation of a similar system developed by these inventors for mineral slurries is presented by P. J. Coghill et. al., Minerals Engineering, 15, 83–90 (2002). In addition, Reibel, U.S. Pat. No. 4,706,509, discloses a similar method featuring use of a plurality of discrete frequencies preferably selected based upon the range of anticipated particle sizes, and measures particle size concentration in the preset ranges by solving a set of linear equations formed from the measured attenuations.
Because of the significance of particle size to many industrial processes, far more patents are concerned with particle size measurements alone. Like the prior art noted above, Uusitalo, U.S. Pat. No. 4,412,451, and Cushman, U.S. Pat. No. 3,779,070, describe the use of attenuation at two discrete frequencies to monitor mean particle size and percent solids in a slurry. However, many other inventors have made use of multiple attenuation measurements covering a wide frequency range. To determine particle size distribution, these researchers have used size-dependent theories for attenuation and then compared these to actual measurements. These theories apply to very small particles where the attenuation is dominated by viscous and thermal effects. Relevant patents covering methods for size distribution based upon analysis of data points from a broad range of discrete frequencies are Dukhin, U.S. Pat. No. 6,109,098; Alba, U.S. Pat. No. 5,121,629; and Reibel, U.S. Pat. No. 4,706,509. The distributions are determined by assuming a starting particle distribution, predicting attenuation at each discrete frequency from the theory, comparing the predicted attenuations with actual attenuation measurements, adjusting the distribution and then repeating this procedure until a suitable match is achieved between prediction and measurement. This method has the disadvantage that a large number of physical properties of the particles and suspending medium must be known to make the predictions, especially for small particles (see McClements, D. J., Langmuir, 12, 3454–3461 (1996)) In addition, the numerical computations required to calculate the theoretical attenuations are very time consuming, and often unstable. These same disadvantages apply to the linear system inversion method of Reibel discussed above. Another complication is the lack of a suitable “multiple scattering” theory needed for dense slurries where the concentration exceeds about 5% volume concentration (see Riebel and Loffler, Part. Part. Syst. Charact. 6, 135–143, 1989). In addition, all of these theories assume spherical particles, and this is certainly not the case for most industrial slurries.
The use of ultrasonic velocity measurements for composition measurement alone is also well known. Such methods can be applied to suspensions and emulsions without concurrent measurement of particle size. Condreva, U.S. Pat. No. 6,295,873, describes a single-frequency transducer, “pulse-echo,” method to detect a single contaminant added to a liquid sample held in a vessel. Changes in the transit-time of the ultrasonic waves are used to sense the presence of the contaminant without providing concentration measurements. Cobb, U.S. Pat. No. 5,473,934, describes a method and apparatus for monitoring the composition of a liquid/liquid or liquid/solid mixture flowing inside a process conduit. The above patent features the use of times of ultrasonic waves traveling through the clamp-on sensor mounts and conduit walls in conjunction with a calibration equation to provide component fractions of these binary mixtures. Urmson, U.S. Pat. No. 5,060,507, describes a different approach that relies on single-frequency resonances of a special chamber to provide composition measurement of a fluid sample. Tavlarides, U.S. Pat. No. 4,852,396 uses single-frequency sound velocity measurement to determine the ratios of two liquids contained in a reactor. Although sufficient for binary mixtures, none of these methods can provide composition information for three or more components. This is especially true when one of the components is a solid or liquid particle dispersed in one or more liquids.
Those skilled in the art readily understand the wide range of potential applications for composition and particle sizing measurement in various industries such as food, mining, pharmaceuticals, chemicals and petroleum. Product composition is an important indicator of quality for many industrial processes and must be monitored and controlled. The present invention is directed toward overcoming one or more of the problems discussed above.